The present invention relates to a device for polishing the surface of a wafer which is roughened due to repeated film forming and etching.
On an LSI (Large Scale Integrated circuit) production line, the surface of a wafer or semiconductor substrate is repeatedly subjected to photolithographic patterning for forming a device. The repeated patterning produces fine irregularities corresponding to the device configuration on the wafer surface. This is particularly true when the designed device size is 1 .mu.m or less. Because the irregular wafer surface makes it difficult to form a wiring layer thereon, it must be flattened. It has been customary to use a wafer polishing device for polishing the wafer surface chemically and mechanically. The conventional polishing device has a rotary holder for holding the rear of the wafer. A turn table faces the holder and has a polishing pad for polishing the front of the wafer held by the holder. A polishing liquid is fed from a nozzle to the pad. A pressing mechanism presses the wafer against the pad via the holder.
However, the conventional polishing device with the above construction has the following problems (1)-(4) left unsolved.
(1) The wafer surface cannot be flattened beyond a certain limit. Why the wafer surface can be flattened by polishing is that the pad contacts the convex portion of the surface with a pressure higher than the pressure with which it contacts the concave portion of the same, thereby polishing the convex portion at a higher rate than the concave portion. In addition, in the convex portion, a broad pattern is difficult to polish. Hence, the wafer surface will become more flat when the pad is implemented by a material having higher rigidity. However, when the pad is formed of an urethane resin or similar material whose rigidity is low, the flattening degree is limited depending on the width of irregularity, and is about 0.5 mm at the present stage of development. On the other hand, in parallel with the progress of fine and large scale LSI technologies, some products recently developed require further enhancement of the flatness of the wafer surface. Particularly, a 64-bit microprocessor or similar large scale circuit results in irregularities as wide as several millimeters and above. It is difficult to flatten such a broad irregularity with the conventional polishing device. While the design of LSI patterns may be so restricted as to obviate broad irregularities, this kind of scheme complicates the design and thereby increases the designing cost and time.
(2) In the case of a wafer of the kind easy to deform itself, i.e., warp or give at its center, it is difficult to remove the irregularities resulting from the formation of a device. The deformation particular to this kind of wafer is ten times to a thousand times as great as the irregularities ascribable to the formation of a device. Therefore, it is likely that a device formed on the convex portion of the wafer is shaved off. The pad may have its elasticity or rigidity lowered to some degree in order to follow the inherent deformation of the wafer, as proposed in the past. However, a decrease in the rigidity of the pad directly translates into an increase in the softness of the pad, and therefore in the polishing ability for removing the irregularities of a device. In any case, the polishing ability cannot be enhanced unless both the elastic strength or rigidity and the softness of the pad which are contradictory to each other are satisfied. However, because the wafer is deformable in various manners, preparing numerous kinds of pads each having a particular rigidity in order to cover all kinds of deformations is not practical when it comes to the actual production line. Particularly, it is impossible to flatten a wafer having irregularities due to its uneven thickness although its deformation is small.
(3) The polishing device consumes a great amount of polishing liquid. The wafer is often scratched unless a polishing liquid constantly fills the gap between the wafer and the pad. In light of this, the pad is provided with a porous structure having bubbles on the surface and in the inside, so that the liquid can easily penetrate into infiltrate into the pad. This makes it difficult for the pad to retain the liquid. As a result, more than a necessary amount of liquid must be fed during the course of polishing, increasing the cost. In addition, the excessive amount of liquid softens the pad and thereby aggravates the above problem regarding the elastic strength of the pad.
(4) The polishing ability available with the conventional device cannot remain stable over a long period of time. When the device polishes the wafer, polishing particles come off the pad and the waste of the wafer deposit to the surface of the pad. This degrades the polishing ability and thereby prevents an even polished surface from being achieved. For this reason, it is necessary to dress the polishing surface of the pad during the interval between consecutive polishing operations. However, the dressing shaves off not only the deposits but also the pad itself, and reduces the thickness of the pad. Consequently, the polishing ability and service life of the pad are reduced. In addition, if the dressing is not even, it is difficult for the pad to flatten the wafer surface to be polished.